Topic 4: CAI Part 2

Type of report which provides an assessment of the strength to be attached to findings

One

Prosecutors fallacy

Primary, secondary or teritiary

Persistance

A weak level of support

• Probability (perhaps think likelihood) is a measurement of that event will occur • Probability is quantified as a number between zero and one • Zero indicates impossibility of the event a occurring • One indicates certainty

The probability of any event will always be a number from 0 to 1

When an event cannot occur, the probability will be zero

When an event is certain to occur, the probability is one

The sum of the probabilities of all the outcomes in the sample is one

The probability that an event will not occur is equal to 1, minus the probability that the event will occur

• ‘E’ the event whose probability is being considered and • ‘I’ the information available to the assessor when the probability of ‘E’ is being considered

• All the probabilities most add up to 1 (as we’ve discussed previously ) • The accused is either guilty or innocent (in England and Wales at least) • pG (Guilty) + pI (innocent) must equal 1

Probability can be either: * Objective - A logical measure of chance where everyone would be expected to agree to the value of the relecant probability * Subjective - Measuring the strength of a persons belief in a particular proposition

X represents variable of interest

P represents the probability of X

The probability that the accused is guilty, in light of the evidence introduced at trial. Typically this is expressed as: 1. p (GIE) or the probability of guilt, given the evidence and 2. p (IIE) or the probability of innocence given the evidence

• Arrested in 2001 after the death of a baby in her care at a hospital in The Hague, apparently from poisoning. • Afterwards, investigators found what they thought was a trend of suspicious deaths among 13 patients treated by De Berk in the previous four years. Five others almost died in what investigators said were suspicious circumstances. • In 2003, she was convicted of four murders and three attempted murders, and sentenced to life in prison. • Part of the evidence against her was the testimony of a statistician, who said the odds were 342 million-to-one that it was a coincidence she had been on duty when all the incidents occurred

• Initially the death was thought of as natural causes. • 30+ suspicious deaths were being investigated • The (flawed) logic and only evidence was that the deaths occurred when she was on duty. • A statistician calculated the 1 in 342 million that this could be a coincidence (Elffers 2002) • Gill (2008) recalculated this to be between 1 in 1,000 and 1 in 25. • After six years in prison she was acquitted in April 2010

Assumptions based on the evidence introduced during trial

• Recovered or questioned samples, namely a sample of unknown origin • The control or reference

The purpose of a substrate control can be used to distinguish between the environment and surroundings and enable us to differentiate these against the questioned samples

The relative frequency is calculated against the items observed over a period of time in a laboratory or from all the DNA profiles in some defined population